1. Field of the Invention
The present invention relates to a Dual Polarization Quadrature Phase Shift Keying (DP QPSK) optical modulator.
2. Description of the Related Art
It is required to increase speed and capacity of optical transmission systems, to handle increased communication amount (traffic). Employing coherent communications capable of greater transmission amount comparing with conventional intensity-modulated communications (on off keying) is studied. Of coherent communications, phase key shifting is gathering attention in particular. Of phase shift keying, there is a format called Dual Polarization Quadrature Phase Shift Keying (hereinafter may be referred to as DP QPSK), in which two quadrature phase shift keying (hereinafter may be referred to as “QPSK”) signals are transmitted, carried by two orthogonal polarized lights, respectively. QPSK is researched as a format whereby high-speed and large-capacity optical transmission systems can be realized.
Described in Hiroshi Yamazaki et. al., “Integrated 100-Gb/s PDM-QPSK modulator using a hybrid assembly technique with silica-based PLCs and LiNbO3 phase modulators”, ECOC 2008, IEEE, 2008, Vol. 1, pp. 17-20, is a DP QPSK optical modulator used in DP QPSK optical transmission systems. The technology described in this literature uses hybrid integration technology to integrate two QPSK optical modulators and polarization beam combiners, thereby forming a DP QPSK optical modulator.